Referring to FIG. 1, a diagram of a front end in a conventional system 10 having a perpendicular magnetic medium 12 is shown. A read signal sensed from the perpendicular magnetic medium 12 has a large amount of power around a DC component. In a conventional read channel, a preamplifier circuit 16 in a magneto-resistive (MR) read head 14 and AC coupling in an analog-front-end circuit 18 block transmission of the DC components of the data read from the medium 12. The preamplifier 16 and the analog-front-end circuit 18 remove only a very narrow frequency band around DC of the transmitted signal to avoid a large signal-to-noise (SNR) loss. The resulting DC-free signal shows a sharp frequency response change around DC and is difficult to equalize to a predefined partial response target. To equalize the DC-free signal properly without incurring a significant SNR loss, both a long equalizer target and a long equalizer are commonly implemented. However, the common implementations result in complex and power hungry systems. Alternatively, refilling the lost DC signal (i.e., DC restoration) by feeding back hard decisions from a detector 20 can achieve a similar SNR gain.
Existing solutions to handle the DC restoration problem have a feedback loop that starts from the detector 20 and ends around an analog-to-digital converter (ADC) in the analog-front-end circuit 18. The feedback loop computes and restores the missing DC components before the detector 20.
The existing solutions have an intrinsic problem of having a long delay present inside the feedback loop. Due to an inability to move backward in time (i.e., an anti-causality problem), the feedback delay sets a limit to the SNR gain of existing feedback DC restoration schemes. Furthermore, the feedback delay in the feedback loop creates complex loop behavior that can cause loop instability.